CZ294789B6 - Dry cement modification method - Google Patents

Abstract

The present invention relates to a dry cement modification method, which method is based on the principle of adding dry undissolved ferrous sulfate into cement between a cement storage tank and an outlet packaging device wherein ferrous sulfate is added in loose form with grain size from 0.1 to 0.8 mm, in the form of FeSOi4.Hi2O monohydrate, and in ratio 0.1 to 1 percent by weight into ready cement immediately at the filter outlet located downstream an air separator, whereupon it is successively mixed and homogenized with cement.

Description

(57)

In the dry cement treatment process, which is based on the principle of adding dry undissolved ferrous sulphate to the cement between a cement mill and a cement silo or an exit packing machine, the ferrous sulphate in a bulk form having a grain size of 0.1 to 0.8 mm in the form of FeSO monohydrate ₄ .H ₂ O is added in a ratio of 0.1 to 1 wt%. into the finished cement directly at the outlet of the filter located downstream of the wind screen, whereupon it is gradually mixed and homogenized with the cement.

CD what σ> what h *

CZ 294

Method of dry cement treatment

Technical field

The invention relates to a method of treating dry cement.

BACKGROUND OF THE INVENTION

It is known that cement containing a chromium compound, when mixed with water, has adverse health effects on persons coming into contact with a mixture of such cement and water. Occurrence of skin eczema is caused by the presence of Cr ⁶⁺ chromium. There are several ways to reduce Cr ⁶⁺ levels in cement-water mixtures. The best known of these is the addition of ferrous sulphate FeSO _fourth H ₂ O during the production of cement, where iron ions of ferrous sulfate reduce hexavalent Cr ⁶⁺ to trivalent Cr ³⁺ , which is only slightly soluble in cement-water mixtures. Cement with the addition of ferrous sulfate is generally known and has been described in a number of patents. For example, U.S. Pat. No. 3,425,892 discloses cement containing 0.1 to 5% ferrous sulfate (the purpose of adding ferrous sulfate was different here, to slow the solidification of the cement after processing). The literature describes the addition of ferrous sulphate to dissolved cement (Annex 3 to circular letter 15, published by the European CEMBUREAU Cement Association in Paris on 1 March 1979), which was sprayed together with gypsum in a cement mill. However, the results were not satisfactory, with Cr ⁶⁺ reduction being very limited. Somewhat better, but still impractical results were achieved by spraying it on hot cement at the mill exit. European Patent Applications EP 54314 and EP 160746 (Int. Cl. ³ C 04 B 7/35) disclose the addition of ferrous sulfate to cement in a dry, undissolved state. This method has proven to be widely used in practice, with 0.4 to 0.6% by weight being the optimum amount of ferrous sulphate in dry cement.

The addition of dry, undissolved ferrous sulphate additive to dry cement always depends on the particular cement manufacturing equipment and is carried out in such a way as to minimize the principles of the existing technology while maintaining the quality and the lowest cost. Although it is generally known that ferrous sulfate can be metered into dry cement in a portion of the process between the mill outlet and the ready-to-store silane (a particular site is not exactly determined), in practice, ferrous sulfate is typically added at the inlet of the cement mill. However, this is disadvantageous because its dosage is not accurate enough. If ferrous sulphate is overdosed, for example in an amount of 1% by weight or more, problems may occur in the processing of concrete made of such cement, and its setting time is considerably prolonged.

The purpose of the present invention is to achieve a dry cement treatment wherein the addition of dry, undissolved ferrous sulfate is dosed as accurately as possible with immediate response to the amount of cement produced. Another requirement is the economy of the solution in order not to interfere with the existing technology of cement production.

SUMMARY OF THE INVENTION

Said objects of the invention are achieved by a dry cement treatment method based on the principle of adding dry, undissolved ferrous sulphate to the cement between the cement mill and the exit cement silo or packaging machine. SUMMARY OF THE INVENTION The invention is based on the fact that ferrous sulphate in bulk form with a grain size of 0.1 to 0.8 mm in the form of FeSO ₄ monohydrate. H ₂ O is added in a ratio of 0.1 to 1 wt. into the finished cement directly at the outlet of the filter located downstream of the wind screen, whereupon it is gradually mixed and homogenized with the cement. Among the benefits

In particular, a very precise dosage of ferrous sulphate, which is given by a substantially instantaneous dosage response, in the order of tenths of a second, to the amount of cement produced can be mentioned in particular. In no case is there an overdose of ferrous sulphate, which could have undesirable effects when processing concrete from the cement thus produced.

The ferrous sulphate preferably has a grain size of up to 0.35 mm. After mixing with the cement, the resulting mixture is more homogeneous.

Ferrous sulphate is added in an amount of 0.4 to 0.6% by weight, this amount being related to the Cr ⁶⁺ content of the cement-containing components, in particular clinker. Ferrous sulphate is mixed with dry cement by vibration and mechanical mixing, possibly in combination with centrifugal spreading. The ferrous sulfate is mixed and homogenized with dry cement in an air stream. This gradual mixing over the entire conveying path from the exit from the cement mill to the packing equipment or cement silo leads to a homogenous homogenization of the cement and ferrous sulphate. At the same time, this solution is very economical, there is no need for any intervention in the existing cement production technology.

DETAILED DESCRIPTION OF THE INVENTION

Ferrous sulphate monohydrate FeSO ₄ . H ₂ O is a fine crystalline substance of gray color with a small amount of accompanying elements (up to 1.0% magnesium and about 0.4% manganese). In water it partially dissolves to solutions with an acid reaction, it is stable at normal temperature. It has a specific gravity of 2650 kg / m ³ , a bulk density of 1270 to 1300 kg / m ³ , a tap density of 1400 to 1800 kg / m ³ (the exact value depends on the fraction). The granulometric composition is such that it contains 20 to 30 wt. % of particles smaller than 0.1 mm, 45 to 65 wt. % of particles having a size of from 0.10 to 0.25 mm, 10 to 20 wt. % of particles having a size of 0.25 to 0.35 mm and at most 1.0 wt. particles larger than 0.35 mm.

In the manufacture of cement, ferrous sulfate monohydrate is fed from the process container by means of a dosing scale which is electronically connected to the instantaneous weight of the finished dry cement produced per strip of finished product, i.e. cement. From the dosing scale, ferrous sulphate with a grain size of up to 0.35 mm is directed to a screw conveyor through which it is conveyed over one or two belt conveyors of finished cement from the outlet of the filter downstream of the wind screen. Here, 0.4% by weight of cement is added to the dry cement. (in alternative examples in a ratio of 0.5 or 0.6 wt.%). Mixing of ferrous sulphate with cement occurs already on the conveyor belt by its movement and vibrations. Further mixing takes place in a rotary cement weighing machine that follows the finished cement belt conveyor. Iron-sulphate cement falls freely onto the weighing machine rotary plate and is centrifuged from the rotating plate onto the weighing machine walls by gravity and falls into the elevator shaft by gravity. In the elevator there is another mixing of the mixture as it is scooped into the elevator bags. When the mixture is subsequently emptied from the elevator bags into the pneumatic conveyor trough, the mixture is mixed again.

In a subsequent set of pneumatic conveying troughs, the mixture is conveyed to a cement silo. In this case, the mixture flows in the inclined troughs in the air stream on the aerated canvas carriers and is mixed and intensely homogenized by this movement. Additional elevator belt conveyors and pneumatic troughs may be incorporated into the material transport. Further homogenization of the mixture occurs in the cement silo.

Industrial applicability

The invention is useful in the manufacture of cement for construction.

Claims (6)

PATENT CLAIMS Process for the treatment of dry cement, based on the principle of adding dry, undissolved ferrous sulphate to cement between a cement mill and a cement silo or an exit packing machine, characterized in that the ferrous sulphate in bulk form having a grain size of 0.1 to 0.8 mm in the form of FeSO4 monohydrate. H ₂ O is added in a ratio of 0.1 to 1 wt. into the finished cement directly at the outlet of the filter located downstream of the wind screen, whereupon it is gradually mixed and homogenized with the cement. Treatment method according to claim 1, characterized in that the ferrous sulphate has a grain size of 0.35 mm. Treatment method according to claim 1 or 2, characterized in that the ferrous sulphate is added in an amount of 0.4 to 0.6% by weight. Treatment method according to one of the preceding claims 1 to 3, characterized in that the ferrous sulphate is mixed with dry cement by vibration and mechanical mixing. Treatment method according to one of the preceding claims 1 to 4, characterized in that the ferrous sulphate is mixed with dry cement by centrifugal spreading. Treatment method according to one of the preceding claims 1 to 3, characterized in that the ferrous sulphate is mixed and homogenized with dry cement in an air stream.